To end her suffering since she was dying slowly... Cause if I was walking the person I love dying painfully and I couldn't do anything but end their suffering I do that.
and the ability probably needs a sacrifice or Blood or something that wasn't mentioned cause the author didn't wanna bother telling us
╮(￣▽￣)╭.
Trust me it gets even more pointless going forward
Yeeeyyy (￣∇￣||)
You so cute ╥﹏╥
Spoiler......
...if he had the ability to conveniently turn back the time...
And start over......why the hell did he had to cut her with his own fucking sword